Few astronauts understand the challenges of zero gravity better than Whitson. The former ISS commander, who retired from NASA on Friday, has spent a total of 665 days with her body weightlessly suspended over the Earth — that's more time off this planet than any other American can boast. She told Business Insider that there's one body part that has to be stronger than the rest to survive in the orbiting office-slash-campsite that is the ISS.

"We go everywhere on that space station with our hands," Whitson said. "Usually, we're traveling at least half a football field's length."

Peggy Whitson suited up for her 200th space walk. NASA

She said astronauts' arms become as essential as Earth-bound humans' feet for getting around in space.

Holding on in the void

Hand and arm strength becomes even more crucial when astronauts venture outside the ISS.

"Everything's very hand intensive, so your hands do have a tendency to get very tired," Whitson said.

She should know: Whitson has spent a mind-boggling 60 hours and 21 minutes outside the ISS on spacewalks, looking into the black void beyond from inside a space suit that would weigh hundreds of pounds on Earth.

"You're in basically your little spaceship built for one," she said.

Staying in the big white suit for hours requires a lot of forearm strength and a super-fit upper body.

"Because the suit's pressurized, it's putting pressure on you," Whitson said. "Even just grabbing your hand together requires more strength than it does here on the ground."

To prepare for these challenges, NASA astronauts do a lot of exercising and training before and during their missions.

How to work out like Whitson

On board the space station, Whitson often strapped herself into a zero-gravity-friendly exercise machine. The device can load up to 600 pounds of resistance for various kinds of arm and leg exercises, and it even has a built-in vibration-isolation system so that the pumping, curling, lifting, and squatting does not disturb the delicate science experiments and equipment on the ISS.

Astronaut Peggy Whitson exercises on the Cycle Ergometer with Vibration Isolation System (CEVIS) in the Destiny laboratory of the International Space Station. NASA

To try one, lie down on your stomach and place your hands on the floor under your shoulders, shoulder width apart. Using only your arms to lift your body, rise up just a few inches, until your lower body is off the ground. Only your toes and hands should be touching the floor. (You can modify this posture by keeping your knees and shins down on the floor.)

Then straighten your arms almost all the way up, keeping a micro-bend at the elbows. Finally, lower your body back down into the starting position.